Natural ventilation setpoints with DOAS HVAC system

Hello everybody,
first of all a happy new year!

I am creating a simple shoebox model to simulate the impact of material choice on the heating and cooling demand through thermal mass (e.g. Wood vs. Concrete). The office building is located in Germany and will have radiant heating and cooling system as well as mech. ventilation for the fresh air demand.
I am looking into the natural ventilation component in more detail and don’t quite understand it.
To test it, I turned the mech. ventilation off and added a schedule to the natural ventilation for daytime ventilation.
As shown in the uploaded picture, I have chosen min and max indoor temperature boundaries for the ventilation, which are within the setpoints for heating and cooling. I added a graph for one week in February. The indoor air temperature clearly falls under 21°C (set_heat) which leads me to the assumption, that the windows are open, even if the temperature drops below the minimum indoor air temp.
If that is correct, the main question is: Why are the windows open when the indoor temperature is not in the boundary?

Can you please help me with that? I’m probably just missing something obvious.

Hey @NicoE ,

You’ve got a pretty sophisticated model there and it’s a tough to know exactly what’s going on. I recommend checking the nat_vent_load output of the HB Read Room Energy Result component since the energy gained/lost there is exclusively for the window ventilation. That will enable you to see if the windows actually are open during that hour or if the temperature is changing for some other reason.

Radiant systems are really slow to change temperature and so I think it’s possible that your indoor temperature variation is more the result of the radiant system than the natural ventilation.

Dear @chris, thank you for your response. It is a good suggestion to look into the nat_vent_load in more detail.
As shown in the picture, the natural ventilation loss is the reason for the temperature drop and the windows are open while the indoor temperature is under the min_in_temp_ (20°).
Am I getting it wright, that the windows are normally only open within the given range of min and max_in_temp_ ?

I have a theory about what might be going on here. I think the windows are normally supposed to open in response to the indoor air termperature, though the E+ input/output reference is not clear about this:

But maybe all of the sophisticated controls for the radiant system are messing with what counts as the zone temperature since the setpoints of these systems tend to be linked to something other than air temperature (like a MRT sensor or a temperature sensor embedded within the radiant slab).

Are you able to recreate this case using a HVAC system that isn’t a Low Temperature Radiation system? That would help us identify if this is part of the issue.

Sorry for the late answer. I tested your theory and changed the HVAC system to a simple HB IdealAir system. And you are right, it is working perfectly with that component! Still, the results of the radiant system confusing me.

1 Like